As described in the parent case, the measurement of the mass flow of a fluid, particularly fluids of varying unknown density, is a difficult task. This difficulty is greatly increased when the line pressure of the fluid is unknown and variable. Line pressure fluctuation is particularly troublesome in flow meters which utilize strain gages on a tube wall, such as those illustrated in U.S. Pat. No. 2,811,855. Line pressure variation of the fluid causes both the hoop stress and longitudinal stress at the tube wall to vary. Complex pressure correction algorithms become necessary in order to accurately compensate the perceived flow measurement for error resulting from line pressure effect.
Another problem frequently encountered in flow measurement is properly sizing the flow meter for a particular flow range likely to occur in a specific application. A flow meter design for very low flows would typically cause very high flow restriction if operated at high flow. On the other hand, a flow meter with a very high maximum flow capability would tend to be very inaccurate when used at flow levels dramatically less than the maximum rated capacity. This useful range is sometimes expressed in terms of turndown ratio. A flow meter with a maximum rated capacity of a 1000 pounds per minute having a turndown ratio of 10 could be used to measure flows in the 100 to 1000 pound per minute range. To measure more than or less than the designed flow rates, an alternative flow meter must be utilized.